Thanks for responding, @Yaobing.
It is definitely editable. XMLTV ID is now shown in its own column in the list of channels. The column is in-place editable. Click twice (slow "double-click") in the cell to start editing.
Ah - I had tried regular double click and single click without any luck. I just tried slow double click and got it to work. It looks like it has to be a really slow double click. I see that the same thing works for most of the other columns in the grid, so this is good to know. It might be nice to add a note somewhere on the window mentioning this functionality as the slow double click to do it isn't immediately obvious (at least to me...).
Alternatively you can manually run EPG loading once. On the page where you see channel matching, find the new channel, and click "Click here to configure", and select the correct entry from the drop-down list.
I actually ended up taking this approach last night after posting my message. I had initially named the channel slightly differently than what was in the guide feed, so an auto match wouldn't have worked. After renaming the channel name to exactly match what's in the guide data feed, it automatically matched it when I manually ran the guide update. For future reference, would the automatic daily guide download have taken care of this after ensuring the channel name was an exact match?